Bacteriophages
Bacteriophages are bacterial viruses that attach to their specific hosts and kill them by internal replication and bacterial lysis involving a complex lytic cycle involving several structural and regulatory genes. Phages are very specific in that they only attack their targeted bacterial hosts. They cannot infect human or other eukaryotic cells. Bacteriophages were first identified, in the early part of the 20th century by Frederick Twort and Felix D'Herelle who called them bacteriophages or bacteria-eaters (from the Greek phago meaning to eat or devour). Duckworth (1976) Bacteriol Rev 40(4): 793-802; Summers (1999) Bacteriophage discovered. Felix d'Herelle and the origins of molecular biology. New Haven, Conn., Yale University Press: 47-59.
Lytic and Lysogenic Bacteriophages
Bacteriophages have a lytic cycle or a lysogenic cycle, but few bacteriophages are capable of carrying out both. With lytic phages such as the T4 phage, bacterial cells are broken open (lysed) and destroyed after immediate replication of the virion. As soon as the cell is destroyed, the new bacteriophage viruses can find new hosts. Kutter and Sulakvelidze (2005) Bacteriophages: Biology and Application. CRC Press: 381-436.
In contrast, the lysogenic cycle does not result in immediate lysing of the host cell. Those phages able to undergo lysogeny are known as temperate phages. Their viral genome will integrate with host DNA and replicate along with it fairly harmlessly, or may even become established as a plasmid. The virus remains dormant until host conditions deteriorate (e.g., due to depletion of nutrients) then the endogenous phages (known as prophages) become active. At this point they initiate the reproductive cycle resulting in lysis of the host cell. As the lysogenic cycle allows the host cell to continue to survive and reproduce, the virus is reproduced in all of the host cell's offspring. See Kutter and Sulakvelidze (2005) Bacteriophages: Biology and Application. 
Bacteriophage Structure
Although different bacteriophages may contain different materials they all contain nucleic acid and protein. Depending upon the phage, the nucleic acid can be either DNA or RNA but not both, and it can exist in various forms. The nucleic acids of phages often contain unusual or modified bases. These modified bases protect phage nucleic acid from nucleases that break down host nucleic acids during phage infection. The size of the nucleic acid varies depending upon the phage. The simplest phages only have enough nucleic acid to code for 3-5 average size gene products while the more complex phages may code for over 100 gene products. The number of different kinds of protein and the amount of each kind of protein in the phage particle will vary depending upon the phage. The simplest phage have many copies of only one or two different proteins while more complex phages may have many different kinds. The proteins function in infection and to protect the nucleic acid from nucleases in the environment. See also McGrath and van Sinderen (2007)
Bacteriophage: Genetics and Molecular Biology.
Bacteriophage come in many different sizes and shapes. The basic structural features of bacteriophages include their size, head or capsid, tail. For example, T4, a common phage is among the largest phages; it is approximately 200 nm long and 80-100 nm wide. Other phages are smaller. Most phages range in size from 24-200 nm in length. All phages contain a head structure which can vary in size and shape. Some are icosahedral (20 sides) others are filamentous. The head or capsid is composed of many copies of one or more different proteins. Inside the head is found the nucleic acid. The head acts as the protective covering for the nucleic acid. Many but not all phages have tails attached to the phage head. The tail is a hollow tube through which the nucleic acid passes during infection. The size of the tail can vary, and some phages do not even have a tail structure. In the more complex phages like T4 the tail is surrounded by a contractile sheath which contracts during infection of the bacterium. At the end of the tail, the more complex phages like T4 have a base plate and one or more tail fibers attached to it. The base plate and tail fibers are involved in the binding of the phage to the bacterial cell. Not all phages have base plates and tail fibers. In these instances, other structures are involved in binding of the phage particle to the bacterium. See Kutter and Sulakvelidze (2005) Bacteriophages: Biology and Application. 
Bacteriophage Infect Bacteria
The first step in the infection process is the adsorption of the phage to the bacterial cell. This step is mediated by the tail fibers or by some analogous structure on those phages that lack tail fibers, and it is reversible. The tail fibers attach to specific receptors on the bacterial cell, and the host specificity of the phage (i.e., the bacteria that it is able to infect) is usually determined by the type of tail fibers that a phage has. The nature of the bacterial receptor varies for different bacteria (e.g., proteins on the outer surface of the bacterium, LPS, pili, and lipoprotein). These receptors are on the bacteria for other purposes, and phage have evolved to use these receptors for infection. See Kutter and Sulakvelidze (2005) Bacteriophages: Biology and Application. 
The attachment of the phage to the bacterium via the tail fibers is a weak one and is reversible. Irreversible binding of phage to a bacterium is mediated by one or more of the components of the base plate. Phages lacking base plates have other ways of becoming tightly bound to the bacterial cell.
The irreversible binding of the phage to the bacterium results in the contraction of the sheath (for those phages which have a sheath), and the hollow tail fiber is pushed through the bacterial envelope. Phages that do not have contractile sheaths use other mechanisms to get the phage particle through the bacterial envelope. Some phages have enzymes that digest various components of the bacterial envelope. See also McGrath and van Sinderen (2007) Bacteriophage: Genetics and Molecular Biology. 
Lytic (Virulent) Phage Life Cycle
Lytic or virulent phages are phages which can only multiply on bacteria and kill the cell by lysis at the end of the life cycle.
During the eclipse phase, no infectious phage particles can be found either inside or outside the bacterial cell. The phage nucleic acid takes over the host biosynthetic machinery, and phage specified mRNAs and proteins are made. There is an orderly expression of phage directed macromolecular synthesis, just as one sees in animal virus infections. Early mRNAs code for early proteins that are needed for phage DNA synthesis and for shutting off host DNA, RNA and protein biosynthesis. After phage DNA is made, late mRNAs and late proteins are made. The late proteins are the structural proteins that comprise the phage as well as the proteins needed for lysis of the bacterial cell. See also McGrath and van Sinderen (2007) Bacteriophage: Genetics and Molecular Biology. 
In the Intracellular Accumulation Phase, the nucleic acid and structural proteins that have been made are assembled and infectious phage particles accumulate within the cell.
During the Lysis and Release Phase, the bacteria begin to lyse due to the accumulation of the phage lysis protein, and intracellular phage are released into the medium. The number of particles released per infected bacteria may be as high as 1000.
A common assay for lytic phage is the plaque assay where lytic phage are enumerated by a plaque assay. A plaque is a clear area which results from the lysis of bacteria. Each plaque arises from a single infectious phage. The infectious particle that gives rise to a plaque is called a pfu (plaque forming unit). See Kutter and Sulakvelidze (2005) Bacteriophages: Biology and Application. 
Lysogenic (Temperate) Phage Life Cycle
Lysogenic or temperate phages are those that can either multiply via the lytic cycle or enter a quiescent state in the cell. In this quiescent state most of the phage genes are not transcribed; the phage genome exists in a repressed state. The phage DNA in this repressed state is called a prophage because it is not a phage but it has the potential to produce phage. In most cases the phage DNA actually integrates into the host chromosome and is replicated along with the host chromosome and passed on to the daughter cells. The cell harboring a prophage is not adversely affected by the presence of the prophage, and the lysogenic state may persist indefinitely. The cell harboring a prophage is termed a lysogen. See also McGrath and van Sinderen (2007) Bacteriophage: Genetics and Molecular Biology, herein incorporated by reference in its entirety.
Anytime a lysogenic bacterium is exposed to adverse conditions, the lysogenic state can be terminated. This process is called induction. Adverse conditions which favor the termination of the lysogenic state include desiccation, exposure to UV or ionizing radiation, and exposure to mutagenic chemicals. This leads to the expression of the phage genes, reversal of the integration process, and lytic multiplication. See Kutter and Sulakvelidze (2005) Bacteriophages: Biology and Application, herein incorporated by reference in its entirety. 
At the time bacteriophages were discovered, with the age of antibiotics still in the future, bacteriophages were considered to be a potentially powerful cure for bacterial infections, and they were therapeutically utilized throughout the world during the pre-antibiotic era. The use of phages in humans was found to be very safe; however, phage therapy did not always work and, with the advent of antibiotics that were effective against a broad spectrum of pathogenic bacteria, it gradually fell out of favor in the United States and Western Europe. Several factors, including the lack of a broad understanding of phage biology, the “Soviet Taint,” and inadequate diagnostic bacteriology techniques, contributed to the failure of some of the early phage therapy studies and to the associated decline of interest in phage therapy in the West. Reviewed in more detail in Sulakvelidze, et al. (2001) Antimicrob Agents Chemother 45(3): 649-659 and Summers (2001) Ann Rev Microbiol 55: 437-51. At the same time, phage therapy continued to be utilized in the former Soviet Union and Eastern Europe, where phage therapy still is being used to treat a wide range of bacterial diseases ranging from intestinal infections to septicemia. Comprehensive information about human and veterinary applications of bacteriophages has been recently reviewed by several investigators. See, e.g., Alisky, et al. (1998) J Infect 36(1): 5-15; Summers (2001) Annu Rev Microbiol 55: 437-51; Merril, et al. (2003) Nat Rev Drug Discov 2(6): 489-497; Sulakvelidze & Barrow (2005) “Phage therapy in animals and agribusiness. Bacteriophages: Biology and Applications.” CRC Press: 335-380; Sulakvelidze & Kutter (2005). Bacteriophage therapy in humans. Bacteriophages: Biology and Application. CRC Press: 381-436.
Despite the use of bacteriophage in various practical settings, including the treatment of diseases in various animals, there remains in the art a need for the discovery of novel bacteriophages, selection of optimal bacteriophages for specific practical applications, and identifying methods for using these bacteriophages in several critical areas, including clinical applications, food safety-related uses and environmental decontamination. For example, one significant need concerns the treatment of processed or unprocessed food products to reduce, eliminate or prevent colonization with undesirable bacteria such as pathogens responsible for food-borne illness and food spoilage organisms. A second critical area of need concerns the removal of undesirable bacteria from industrial environments such as food processing facilities to prevent colonization thereof. A third critical area of need concerns the removal of antibiotic resistant organisms from environments where they may be passed to susceptible humans and animals, such as hospitals, nursing homes, veterinary facilities, and other such environments. Additionally, new bacteriophage and methods of using the same are needed for the prevention or treatment of animal and human bacterial disease, particularly those diseases caused by antibiotic-resistant organisms. Finally, bacteriophage compositions may be used a probiotics (e.g., the bacteriophage lyse undesirable bacteria leaving desirable microflora intact).